1. Field of the Invention
The present invention relates to a photomultiplier capable of successively emitting secondary electrons in multiple stages in response to incidence of photoelectrons from a photocathode and thereby performing cascade multiplication of the secondary electrons.
2. Related Background Art
The development of TOF-PET (time-of-flight PET) as a next-generation PET (positron emission tomography) apparatus is being pursued actively in the field of nuclear medicine in recent years. In a TOF-PET apparatus, because two gamma rays, emitted from a radioactive isotope administered into a body, are measured simultaneously, a large number of photomultipliers having excellent, high-speed response properties are used as measuring devices disposed so as to surround a subject.
In particular, in order to realize high-speed response properties of higher stability, multichannel electron multipliers, in which a plurality of electron multiplier channels are prepared and electron multiplications are performed in parallel at the plurality of electron multiplier channels, are being applied to next-generation PETs such as that mentioned above in an increasing number of cases. For example, a multichannel electron multiplier described in International Publication WO2005/091332 has a structure in which a single incidence surface plate is partitioned into a plurality of light incidence regions (each being a photocathode to which a single electron multiplier channel is allocated), and a plurality of electron multiplier sections (each including a dynode unit, in turn including multiple stages of dynodes, and an anode), prepared as electron multiplier channels that are allocated to the plurality of light incidence regions, are sealed inside a single glass tube. A photomultiplier with the structure where a plurality of photomultipliers are contained inside a single glass tube is generally called a multichannel photomultiplier.
A multichannel photomultiplier thus has a structure where a function of a single-channel photomultiplier, in which photoelectrons emitted from a photocathode disposed on an incidence surface plate are electron multiplied by a single electron multiplier section to obtain an anode output, is shared by the plurality of electron multiplier channels. For example, with a multichannel electron multiplier, with which four light incidence regions (photocathodes for electron multiplier channels) are arrayed in two dimensions, because for one electron multiplier channel, a photoelectron emission region (effective region of the photocathode) is made ¼ or less of the incidence surface plate, electron transit time differences among the respective electron multiplier channels can also be improved readily. Consequently, in comparison to the electron transit time differences within the entirety of a single channel photomultiplier, a significant improvement in electron transit time differences can be anticipated with the entirety of a multichannel electron multiplier.